Re: [PATCH] LoongArch: Add Syscall Assembly Implementation

[Xi Ruoyao <xry111@xry111.site>]

On Fri, 2023-03-24 at 01:34 +0800, Xi Ruoyao via Libc-alpha wrote:
> On Thu, 2023-03-23 at 21:43 +0800, Xi Ruoyao wrote:
> > On Thu, 2023-03-23 at 21:34 +0800, Xi Ruoyao wrote:
> > > On Thu, 2023-03-23 at 14:12 +0100, Andreas Schwab wrote:
> > > > On Mär 23 2023, caiyinyu wrote:
> > > > 
> > > > > Without this patch(objdump -d libc.so...):
> > > > > 
> > > > > 00000000000dd45c <syscall>:
> > > > >    dd45c:       02fec063        addi.d          $sp, $sp, -
> > > > > 80(0xfb0)
> > > > >    dd460:       02c0606c        addi.d          $t0, $sp, 24(0x18)
> > > > >    dd464:       29c06065        st.d            $a1, $sp, 24(0x18)
> > > > >    dd468:       29c08066        st.d            $a2, $sp, 32(0x20)
> > > > >    dd46c:       29c0a067        st.d            $a3, $sp, 40(0x28)
> > > > >    dd470:       29c0c068        st.d            $a4, $sp, 48(0x30)
> > > > >    dd474:       29c0e069        st.d            $a5, $sp, 56(0x38)
> > > > >    dd478:       29c1206b        st.d            $a7, $sp, 72(0x48)
> > > > >    dd47c:       29c1006a        st.d            $a6, $sp, 64(0x40)
> > > > 
> > > > If the argument registers are call-clobbbered, why does the compiler
> > > > need to save them?
> > > 
> > > It seems triggered by va_start.  If I don't use "..." and replace it
> > > with "a0, a1, a2, ..., a5", and remove va_start ... va_end, the
> > > compiled
> > > code won't save registers.
> > > 
> > > I'll try to investigate further.
> > 
> > Similar to GCC PR100955.
> 
> Nope, it's not PR100955.  PR100955 is about AArch64 but syscall is
> compiled to almost perfect assemble code on AArch64.

I was wrong.  AArch64 has a assembly syscall.

> It looks like caused by the lack of [TARGET_SETUP_INCOMING_VARARGS][1]
> in GCC config/loongarch.  I'll try to add it...

LoongArch has a TARGET_SETUP_INCOMING_VARARGS but it does not use the
information from stdarg pass.  I can fix it, but even with the fix GCC
would still save 7 registers (now GCC trunk saves 9 registers, the fix
would make some improvement but no much).

And the issue seems not trivial to fix.  On x86_64, all of GCC, Clang,
and MSVC will save some registers if va_arg is used.  I've not found any
compiler which can avoid saving the va_arg GARs unnecessarily yet:

https://godbolt.org/z/n1YqWq9c9

Now to me it seems a bad idea to use va_arg in syscall.c.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University